The influence of creases on carton board package behavior during point loading

1 Introduction

The present study deals with the impact of crease variations on the mechanical properties of carton board packaging at point load. The question is if differences can be seen at point loading of a package due to differences in the creasing geometry and method? Since the properties of the crease are important for the mechanical function of the packaging, both for functional behavior and appearance (Coffin and Nygårds 2017), it may be anticipated to be important also for the packaging’s ability to resist point loads.

The purpose of creasing is usually to reduce the bending resistance when the carton board is folded to shape a package, which facilitates the acquisition of easily filled, shape-stable packages. Upon creasing a ruler is used to make an indent in the carton board against a matrix, see Figure 1. In this paper, the name creasing will be used throughout for all matrix heights, even zero. Previous research shows that different crease appearances and crease properties can be obtained for creased and folded carton board depending on the creasing and folding method (Carlsson et al. 1983; Cavlin et al. 1997; Coffin and Nygårds 2017; Dunn 2000; Giampieri 2009; Hine 1959; Xia 2002). Coffin and Nygårds (2017) gives a historical review of research concerning creasing and folding. The importance of delamination (Bird 1912) and bead formation when the carton board is folded towards the ridge (Shelton De 1876) for good crease performance was early identified. The creasing should initiate damage of the carton board so that several symmetric delamination lines distribute over the carton board thickness under folding and facilitates that a symmetric well-defined bead is formed (Carlsson et al. 1983; Coffin and Nygårds 2017; Dunn 2000; Niskanen 2011; Xia 2002). Both too weak and too hard creasing may give problems such as cracks (Hine 1959). A good crease is characterized by a well-defined remaining straight indentation groove/crease line on the upper surface of the carton board and a well-defined symmetric bead on the inside of the fold. The crease appearance can be coupled to the crease properties, such as bending moments (Carlsson et al. 1983). Different operational windows for creasing have over time been derived specifying creasing parameters that will yield adequate creases and folds (Coffin and Nygårds 2017; Marin et al. 2022). Much research effort has been put on creating numerical models, both of creasing and folding that matches creasing and folding results (Beex and Peerlings 2009; Domaneschi et al. 2017; Huang and Nygårds 2011; Nygårds et al. 2009; Robertsson et al. 2021) and of loading of packages that matches experimental loading results (Marin et al. 2020; Nygårds et al. 2019; Ristinmaa et al. 2012). The experimental studies and modelling of packages show that the mechanical properties of the creases are important for capturing the behavior of the package. Different crease geometries thus produce creases with different properties that affect the behavior of the creased and folded packages. But, can a difference due to crease type be seen when a package is loaded with a point load?

Figure 1:

Schematic arrangement and concepts of creasing equipment.

The aim was to determine if creasing matrix and crease properties affects the properties of the package. The crease appearance was studied in microscope, the crease behavior was measured as residual moment and the characteristic of the package was measured as stiffness and maximum load at point load. The goal was to see if the use of different creasing affects how creases look and behave and affect the characteristics of the packaging as measured by behavior at point load.

2 Materials and methods

To investigate if a difference is visible at point load measurements, carton board was creased with different creasing matrixes. The idea was to change the matrix height to get carton board with creases of different appearance and with different properties. Also, two creasing methods were used, a commercial flatbed die with crease rulers and a CAM table with a creasing wheel. The exact same measures were not obtained for the flatbed die and CAM table matrix, but the dimensions are similar. The appearances of the different creases were studied in microscope both folded and unfolded, and their residual moments upon folding were measured. The residual moment is a measure of the decrease of bending resistance. Low residual moment facilitates folding, but the residual moment should not be too low (Coffin and Nygårds 2017) Creases with too low residual moments generally give packages that are difficult to fill and cause jam in the filling machine and crease cracks. Creases with too high residual moment can lead to largely bulged packages that may also cause jam in the filling machine. The packages made of the creased carton board were point loaded and the force displacement curves analyzed.

The carton board used for the packages had thickness 0.451 mm. It is a four-ply board in which the top ply is bleached sulfate pulp and the two middle plies are a mixture of unbleached mechanical pulp (CTMP) and unbleached sulfate pulp, and the bottom ply is of unbleached sulfate pulp. The plies have been couched together and on the top ply a 3 layer clay coating is applied, see Figure 2. Table 1 gives key material parameters of the carton board.

Figure 2:

Schematic display of the four layers of the carton board along with its clay coating.

The creasing was performed with 4 different crease geometries, according to Table 2. Crease A was performed with a commercial flatbed cut and creaser Crosland VK 1130 with Pertinax matrix, Crease B was performed on a computer aided manufacturing (CAM) table (Kongsberg Esko) without matrix, Crease C was performed on a CAM table (Kongsberg Esko) with matrix of the same quality as the packaging (matrix 1) and Crease D was performed on a CAM table (Kongsberg Esko) with matrix of higher grammage than on the package (matrix 2). On the CAM table different crease geometry was achieved by using a sheet of carton board with precut creasing channels as a creasing matrix. The carton board creasing matrix was placed under the carton board from which the package blank was cut and a creasing wheel was rolled on the paperboard acting as a creasing rule. All creases were performed on a flatbed cut and creaser or CAM table with a 2 point (0.71 mm) creasing ruler or wheel. Crease A with channel width 1.4 mm and channel depth 0.5 mm, Crease B no matrix, Crease C and Crease D with channel width 1.8 mm and channel depth 0.451 mm and 0.528 mm respectively. The fibers in the paper board get oriented during manufacturing and the paper board gets different properties in the machine direction (MD) and cross direction (CD) (see Table 1). The creases that were further studied concerning appearance and bending were MD-creases, which means that the carton board was bent in the MD-direction, see Figure 3.

Figure 3:

MD-crease with directions of the carton board achieved during production given. In the figure the material direction is given by MD and CD.

For analysis of variations in the crease, a crease bend test was performed with a Lorenzen and Wettre PTS Creasability Tester (ABB Lorentzen & Wettre products, Stockholm, Sweden) according to usual procedure. The creased samples were clamped by a rotatable clamp with the crease in the rotational center, see Figure 4. Samples were 50 mm wide with a bending length of 25 mm and bent to 160° and back at a speed of 90°/second. A load cell situated 25 mm above the clamp measured the reaction force during the bending. Graphs showing moment to angle were saved, and the residual moment value calculated at 90° during loading (moment creased/moment uncreased). The residual moment measurement was made at least 3 days after the creasing. 4 samples were performed per crease type.

Figure 4:

Measurement equipment for crease bending test.

The creases were analyzed before and after folding using a scanning electron microscope (SEM) at 50 times magnification. During sample preparation, the carton board was molded into an epoxy cylinder at under pressure, so that the epoxy penetrated pores and cavities. The sample was grinded to the cross-sectional surface that was then studied. SEM pictures were analyzed visually, and dimensions determined by measurements in the SEM pictures using the commercial image processing program Adobe Photoshop 2023.

All measurements of the creases were carried out in standardized and controlled climate (23°, 50 % RH).

The packaging was made of blanks with dimensions according to Figure 5a. At erecting, the blanks were folded 180° and double adhesive tape (Tesa 9 mm, part number 04959) was applied to the flaps. The packaging was erected about a day after the creasing and load tested about 3 days after the rising. The dimensions of the folded packaging were as shown in Figure 5b.

Figure 5:

On the left the blank, on the right the erected package with its dimensions and the measuring position of the point load. The measurement position was in the middle of the long side of the package, marked red, in the upper picture. The first point of contact is illustrated in the lower picture of the indenter mounted in the compression tester.

The packages were loaded with a point load at the center of the long panel (100 mm from the short side), 12.3 mm in from the edge, as shown in Figure 5b. The packages were placed in position by aligning small lines drawn on the center of the package’s long edge and on the table of the measurement equipment. The point load consisted of a finger-like indenter with a diameter of about 14.8 mm (Biotac, Syntouch Inc., 3720 Clifton Pl, Montrose, CA 91020, USA). For the measurement of force versus displacement, the indenter was mounted in a uni-axial compression tester Lloyd LR5K with a speed of 60 mm/min (max load 500N, accuracy 0.5 %). Measurements were performed on 10 packages per crease type. The tests were carried out on the creases alternately one after another. The compression tester was started with the indenter above the carton board package. The indenter moved downwards at the constant speed to a predetermined depth determined so that damage occurred, before returning to the zero level. All the point load measurements on the packages were carried out at a controlled climate (RF 18.5–20.5 %, temperature 21–23°). The collapse load and stiffness were determined from the force displacement curves. The package collapse load is located at the first peak, see Figure 6. The maximum stiffness before collapse load is given by the maximum slope of the curve before the collapse load. It was determined by a moving average over 3 points (0.02 mm), filtered by a five-point moving average filter in the commercial mathematical software Matlab. The reasonableness of the achieved slope was checked manually according to the graph.

Figure 6:

A typical point load curve with the package collapse load (PCL) marked and a line with the slope of the stiffness. Package collapse load (PCL) is the first max point in the force displacement curve. The stiffness is given by the maximum slope before the collapse load.

3 Results and discussion

The appearance of the carton board in SEM after creasing and before folding for Crease A, B, C and D is seen in Figure 7. The appearance after folding is seen in Figure 8. Measured characteristics chosen to reflect the appearance of the creases before and after folding respectively are illustrated in Figure 9 and given in Tables 3 and 4. In the SEM images, the coating was seen as a white layer at the top of the carton board. The air pockets were filled with epoxy and were the same color as the background color (epoxy). It was seen that the plies in the middle were less dense than the outer plies. Some air pockets may be black. Black parts were areas where epoxy had not penetrated or connected optimally.

Figure 7:

The appearance of the carton board in SEM after creasing but before folding for Crease A, B, C and D.

Figure 8:

The appearance of the carton board in SEM after creasing and folding for Crease A, B, C and D.

Figure 9:

Characteristics of creased carton board (left) and creased and folded carton board (right). Before folding. I: Distance from unaffected top of carton board to lowest top of the carton board in the crease/groove. II: Distance from unaffected top to bottom of the creased carton board at the ridge. III: The difference of the two previous numbers, i.e. the thickness of the board at the indentation point. After folding IV: Distance from unaffected top of carton board to lowest top of the carton board in the crease, V: Distance from unaffected top of carton board to bottom of the creased and folded carton board at the bead and VI: Distance between greatest indentation points at the bottom of the carton board.

Before folding the top side of the crease may show a residual indentation from the creasing, measure I, see Figures 7 and 9. The residual indentation was deepest for Crease A where the crease was performed with the commercial flatbed cut and creaser, while measure I was less for Crease B, C and D that were performed on the CAM table. Least measure I had Crease B which was creased without matrix. Measure II follows the same trend, with the highest value for Crease A and the lowest for Crease B. The carton board thickness at the crease, measure III, was lowest for Crease A and highest for Crease D.

After folding, according to measure IV a residual indentation remained for crease A, while the outer ply was stretched for Crease B. For Crease C and D, the indentation was not left, but it was difficult to judge whether the material was stretched or not. None of the creases had visible crease cracks. Crease A exhibited a symmetrical appearance, also in terms of the delamination. Crease B, on the other hand, had an unsymmetrical bead. Crease C and D had similar appearances, which was not entirely symmetrical. Crease C and D were well delaminated, but had a slightly thicker layer without delamination at the top compared to Crease A. While delamination in Crease A ended about at minimum thickness of the carton board, delamination in Crease C and D continued outside the crease area. Crease C and D had a pointed bead while A had a rounded one.

In conclusion, three different appearances of the creases were seen in the SEM pictures; the symmetric and well-delaminated appearance of Crease A, the more pointed and unsymmetric appearance of Crease C and D and the appearance with small and unsymmetric bead of Crease B. Crease C and D were difficult to distinguish in appearance, even as folded. The only thing that distinguished them in the manufacture was 0.077 mm in matrix depth for the 0.451 mm thick carton board. This did not make enough difference to create visually different creases.

The results of the residual moment measurements are shown in Figure 10 and Table 5. Creased samples gave a lower moment than uncreased ones, with residual moment according to Table 5. The bending moments in Figure 10 deviate from that of an ideal plastic material, the samples’ bending moments decrease after max and then begin to rise. This may be explained by delamination respectively obstacles in the extent of damage, such as the bulge reaching the wall. The different creases had different residual moments. Order from lowest to highest residual moment of the creases was Crease A, D, C, B. The most well-defined crease i.e. straight, symmetric and well-delaminated had the lowest residual moment and vice versa. The residual moments of Crease C and D were not identical, but the difference was small. Crease C and D had similar characteristics.

Figure 10:

L&W residual moment measurements for Crease A, B, C and D. The red lines show the average of the four measurements of the moment when bending uncreased samples to 160°. The blue lines show the corresponding results for the creased samples. The moment marked at 90° for all graphs.

Figure 6 shows a typical point load curve. Independent of crease, the packages were able to take about the same maximum point load before damage, see Table 6. Both the average packaging collapse load and average stiffness for the different creases A, B, C and D differed less than the standard deviation of the measurements which means that it is not statistically possible to say that the creases gave any difference in the behavior of the packaging measured as point load in this study. However, the trend of the average stiffness was that a deeper matrix channel depth gave a lower stiffness. The stiffness measurements of Crease A had the lowest standard deviation. This may be caused by that Crease A was made in a different way than Crease B, C and D, using flatbed creasing instead of creasing on a CAM-table. Flatbed creasing performs the cutting and creasing in the same operation which may be expected to result in a more tensioned sample than when a CAM table is used. On a CAM table the creasing and cutting are not performed simultaneously.

4 Conclusions

By varying the creasing matrix and using two creasing methods four creases were made. Since two of the creases had similar appearance and behavior, creases with three distinct characteristics were achieved. The crease, which was carried out with a commercial flatbed cut and creaser, became more symmetrical, with a more marked residual indentation than the creases performed on CAM table, and received a lower residual moment. For this crease the matrix channel depth was between the others studied, but the matrix channel width was narrower. Also, the creasing method with creasing and cutting at the same time may be expected to cause a different tension of the carton board than when the creasing and cutting is not performed at the same time. A general conclusion that conforms with our studied creases is that a more well-defined crease gives a lower residual moment independent of used creasing method.

Despite the different characteristics of the creases, it is not possible to separate the performance of the packages with the different creases during point loading. The differences of the package collapse loads and stiffnesses were within the statistical uncertainty of the measurements. The maximum point load before damage was about the same for the packages. The result showed a trend that creasing with a deeper matrix channel depth gave a lower average stiffness at point loading, but not statistically ensured. This indicate that it may be interesting to study the influence of creases on the stiffness of packages at point loading in further research.

Since different creases were obtained, regarding appearance and residual moment, and creases have been shown to have influence on packages mechanical behavior in previous research for other load cases, the authors expected to see a difference in mechanical performance of the packages at point loading. No statistically ensured difference could however be shown of the package’s behavior. To further investigate possible differences, further measurements would be interesting. The measurement method could be improved to measure the interaction between the package and the indenter more in detail. One possibility could be to study the deformation of the package and the enclosure of the indenter.